Aquatic ecosystem is a hydrological system or environment that comprises of water, aquatic organisms and hydrological processes. This article discusses aquatic ecosystem definition, structure, functions, importance, and examples as outlined below;
Aquatic Ecosystem Definition: 5 Ways to Define Aquatic Ecosystems
An aquatic ecosystem is a bio-geographical environment that is dominated by water, which determines its composition and internal conditions.
The above is a most-basic aquatic ecosystem definition, which highlights the fact that such an ecosystem is dominated by water and mostly influenced by water-related factors.
Further clarity on the concept can be gained by identifying the characteristics of aquatic ecosystems, as is done in the alternative aquatic ecosystem definition below;
Aquatic ecosystem refers to a type of ecosystem that is characterized by water-based physicochemical properties, dominance of microscopic producers, inverted biomass pyramid, highly-adapted biodiversity, and relatively-low solar penetration.
Characteristics like the inverted biomass pyramid  are what make aquatic ecosystems unique and distinct from terrestrial ecosystems like forests. Physicochemical factors, on the other hand, can also be seen as abiotic factors in aquatic ecosystems, and include; pH, turbidity, salinity, dissolved oxygen, temperature and acidity .
Based on its characteristics, different types of aquatic ecosystems can be identified and distinguished. The are listed in the alternative aquatic ecosystem definition that is outlined below;
Aquatic ecosystem is a water-dominated biome with unique characteristics that are used to distinguish it into various types including; marine surface, marine coastal, wetland, lentic and lotic ecosystems  .
Multiple examples of aquatic ecosystems can be given under the types listed above. The following aquatic ecosystem definition highlights some of them;
An aquatic ecosystem is a natural environment surrounded by and/or submerged in fresh or salt water, with examples such as; lake, river, ocean, swamp and wetland .
The above-listed are typical examples of aquatic ecosystems, while other secondary examples include; estuaries, streams, ponds, coral reefs.
Lastly, some animals in aquatic habitats are mentioned as part of the aquatic ecosystem definition;
Aquatic ecosystem is a water-dominated terrain that is inhabited by organisms like; crustaceans, fish, amphibians, algae, soft-bodied invertebrates (like echinoderms and mollusks), aquatic reptiles and mammals.
Structure of Aquatic Ecosystem
The structure of aquatic ecosystem comprises of three units; surface, near-surface, and deep water zones. These components are found in both freshwater and marine ecosystems.
They also help to distinguish the various trophic levels of the aquatic ecosystem food chain, each of which is closely related to one or more of the structural components of an aquatic ecosystem.
In addition, biotic and abiotic factors must also be defined when discussing the structure of aquatic ecosystem, as shown below;
1). Surface Zone (as part of the Structure of Aquatic Ecosystem)
In general, aquatic systems are segmented into four zones that are named in order from surface to bottom, as; littoral, limentic, profundal and benthic zones .
When discussing the surface zone as part of the structure of an aquatic ecosystem, we restrict our focus and assessment to the littoral zone.
This zone includes the area of land surrounding the water body of an aquatic ecosystem, and can be said to span from the shore to water margin.
Basically, the surface zone refers to all parts of an aquatic ecosystem that are not always submerged under water.
The surface zone is very important toward shaping the entire structure of an aquatic ecosystem, because sedimentary processes like erosion and deposition occur within this zone. Such processes can alter the quality of water in the ecosystem, and can affect the shape of the water basin, as well as the total areal coverage of the ecosystem; also called the watershed area.
Some aquatic organisms also use the surface zone as a primary or alternative habitat, including some aquatic plants, crustaceans, reptiles and amphibians.
Important biotic and abiotic factors within the surface zone of an aquatic ecological structure include; temperature, air pressure, air quality, humidity, soil pH and acidity.
The surface zone is unique for allowing a significant amount of solar penetration, so that it records the highest rate of photosynthesis across the entire structure of aquatic ecosystems, and is often called the 'photic' zone .
2). Near-Surface Zone
The near-surface zone of an aquatic ecosystem is also referred to as the neritic region or zone, and covers both limnetic and profundal zones in general depth-classification.
Solar penetration is also significant in the near-surface zone, although less than the surface zone. Asides this, the surface and near-surface can be distinguished by the fact that the near-surface is mostly submerged in water, while the surface zone is not.
With regards to aquatic trophic levels, the near-surface mainly comprises of Level 1 processes and organisms. These processes are mostly photosynthetic in nature, and the organisms include shallow-water autotrophs like phytoplankton.
Other shallow-water, heterotrophic organisms like zooplankton may also occur in the near-surface zone. While being incapable of producing their own food, these organisms are dependent on the autotrophs, so that they act as primary consumers in the aquatic energy pyramid.
Abiotic factors that are of importance in the near-surface zone are; temperature, pH, salinity, acidity, turbidity, and light penetration.
3). Deep-Water Zone (as part of the Structure of Aquatic Ecosystem)
The deep water zone is also called the benthic zone, in reference to the bottom-dwelling organisms which may be found at depth in an aquatic ecosystem.
In marine biomes, the deep water zone is referred to as 'abyssal zone', and is estimated to occur from 4,000 m depth, downward .
Biologically, the deep water zone is the most inactive component of the aquatic ecosystem structure. This is due to relatively low temperatures and low light penetration at such depth. In smaller aquatic ecosystems like lakes and ponds, aquatic plants and some animals may survive at the bottom.
Various processes occurring in the deep water zone, contribute to the overall structure of aquatic ecosystems.
Functions of Aquatic Ecosystem
Functions of aquatic ecosystem are;
1. Domestic water supply
2. Climate modification
3. Sedimentation and landform change
4. Nutrient cycling
7. Organic habitat provision
8. Waterborne transport
Importance of Aquatic Ecosystem
The importance of aquatic ecosystem is based on its role in the hydrological cycle, energy provision, nutrient cycling and habitat provision for Earth's organisms, as well as the modification of landscapes and formation of minerals and sedimentary rocks.
Below is an outline of the importance of aquatic ecosystems;
1. Food supply
2. Habitat provision
3. Energy production
4. Geochemical cycling
5. Landscape modification
6. Rock and mineral formation
Examples of Aquatic Ecosystems
Examples of aquatic ecosystems are;
8. Coral Reefs
Aquatic ecosystem is an environmental and biological system that is submerged in, or surrounded by, water, and whose organisms and characteristics are strongly dictated by hydrological factors.
The structure of aquatic ecosystem comprises of;
1. Surface Zone
2. Near-Surface Zone
3. Deep-Water Zone
Functions of aquatic ecosystem are; domestic water supply, climate modification, sedimentation and landform change, nutrient cycling, renewable energy production, hazard mitigation, organic habitat provision, and waterborne transport.
The importance of aquatic ecosystem includes its roles in; food supply, habitat provision, energy production, geochemical cycling, landscape modification, rock and mineral formation.
Examples of aquatic ecosystems are freshwater systems like; rivers, streams, wetlands, lakes; and brackish to saltwater systems like; swamps, oceans, estuaries, and coral reefs.
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